Stacked active region laser array for multicolor emissions

ABSTRACT

Monolithic arrays having closely spaced laser stripes which output laser beams with large, but well-controlled, wavelength separations. The monolithic array uses a plurality of stacked active regions which are stacked in the order of decreasing energy bandgaps as one moves away from the substrate. Those active regions are separated by one or more thin etch stop layers. Between the bottom active regions and the substrate is a lower cladding layer, while over the topmost active region of each stack is an upper cladding layer. Beneficially, an electrical connection is made to each stack using a heavily doped capping layer/metallic contact above each stack and a metallic contact on the substrate (which is shared by all stacks). Lateral carrier and optical confinement is achieved using a confinement layer which surrounds each stack. Beneficially, that confinement layer is formed using layer induced disordering.

This invention relates to solid-state laser arrays which output multiplelaser beams having different colors (wavelengths).

BACKGROUND OF THE INVENTION

The performance of many devices, such as laser printers and opticalmemories, can be improved by the incorporation of multiple laser beams.For example, laser printers which use multiple beams can have higherprinting speeds and/or better spot acuity than printers which use only asingle beam.

In many applications, closely spaced laser beams of different colors(wavelengths) are desirable. For example, color printers which useclosely spaced laser beams of different colors can overlap the beams,sweep those overlapping beams using a single raster output polygonscanner and a single set of optics, subsequently separate the individualbeams using color selective filters, direct each beam onto a separatexerographic imaging station, develop a latent image for each color on adifferent recording medium, and produce a full color image bysequentially developing each latent image on a single recording medium.

One way to obtain closely spaced laser beams is to form multiple laseremission sites, or laser stripes, on a common substrate. While thisenables very closely spaced beams, prior art monolithic laser arraystypically output laser beams at only one color.

However, various techniques are known in the prior art for producingdifferent color laser beams from a monolithic laser array. For example,it is well known that a small amount of color difference can be obtainedby varying the drive conditions at each lasing region. However, theeasily achievable color difference is insufficient for mostapplications.

One method of achieving large wavelength separations is to grow a firstset of active layers on a substrate to form a first lasing element whichoutputs light at one wavelength, and then to form a second set of activelayers next to the first to form a second lasing element at a secondwavelength. However, this method requires separate crystal growths foreach lasing element, something which is not easily performed.

Another technique for obtaining different color laser beams from amonolithic laser array is to use stacked active regions. A stackedactive region monolithic array is one in which a plurality of activeregions are sandwiched between common cladding layers. Each activeregion is comprised of a thin volume that is contained within a laserstripe. The laser stripes contain different numbers of active regionsthat emit laser beams at different wavelengths. Several stacked activeregion structures are discussed in U.S. Pat. No. 5,157,680, entitled"Integrated Semiconductor Laser," issued 20 Oct. 1992 to Goto.

In a stacked active region monolithic laser array, current flows inseries through the stacked active regions. The active region with thelowest bandgap energy will lase, thereby determining the color of thelaser beam output from that part of the array. To provide another coloroutput, the previously lowest bandgap energy active region is removedfrom part of the array and current is sent through the remaining stackedregions.

Stacked active region monolithic laser arrays can not only outputclosely spaced laser beams of different colors, but beneficially theoutput laser beams are axially aligned with each other (share the sameoptical axes). In practice, the stacked regions of a stacked activeregion monolithic laser array are very closely spaced; separations inthe stack direction typically being about 100 nm.

A big problem with stacked active region monolithic laser arrays is thatthey have been difficult to fabricate, particularly in the AlGaAsmaterial system. This is at least partially because the proper stackedactive regions must be formed in each part of the structure.Conceptually, this problem can be solved by simply growing planarepitaxial layers which contain the required active regions such that thebandgap energies of the active regions decrease as one moves towards thecrystal surface. Then, one could simply remove active regions, asrequired, to obtain the desired wavelength from each region of thearray. Finally, the required cladding layer and capping layers are grownover the remaining active regions.

However, it is very difficult to precisely etch the areas between theactive regions when those active regions are closely spaced. Further,because of undesired growths on many materials when those materials areexposed to air, such as oxide growths on some compositions of AlGaAs, itis very difficult to achieve the required high quality growths over theremaining active regions. Thus, the simple conceptual approach givenabove is difficult to implement in some material systems, for examplethose containing aluminum.

Therefore, it would be useful to have stacked active region structurescapable of outputting closely spaced, multiple color laser beams. Itwould also be useful to have techniques for producing stacked activeregion structures capable of outputting closely spaced, multiple colorlaser beams in material systems which are subject to undesired oxidationupon exposure to the atmosphere prior to growth of overlayers.

SUMMARY OF INVENTION

An object of the invention is a stacked active region structure capableof outputting closely spaced, multiple color laser beams.

Another object of the invention is to produce closely spaced, multiplecolor laser beams in a device which is nearly planar.

Another object is a technique for producing stacked active regionstructures capable of outputting closely spaced, multiple color laserbeams.

Yet another object is a technique for producing stacked active regionstructures which requires only two epitaxial crystal growth steps.

A further object of the invention is a device capable of producingclosely spaced, multiple color laser sources that are axially alignedwith each other.

The invention relates to monolithic arrays, and techniques forfabricating such arrays, which have closely spaced laser stripes andwhich output laser beams with large, but well-controlled, wavelengthseparations. The monolithic array has a plurality of stacked activeregions which are stacked in the order of increasing energy bandgaps asone moves toward the substrate. Those active regions are separated byone or more thin etch stop layers. Between the bottom active regions andthe substrate is a lower cladding layer, while over the topmost activeregion of each stack is an upper cladding layer. Beneficially, anelectrical connection is made to each stack using a heavily dopedcapping layer/metallic contact above each stack and a metallic contacton the substrate (which is shared by all stacks). Lateral carrier andoptical confinement, for enhanced performance, is achieved using aconfinement layer which surrounds each stack. Beneficially, thatconfinement layer is formed using impurity-induced layer disordering.

A method of fabricating such monolithic arrays is to first fabricate allactive regions (in order of wavelength), lower cladding layer, and etchstop layers on a single substrate. Then, using patterned etching, thevarious active layers are removed (etched) from selected portions of thestructure such that the top active region that remains in each stackwill output the desired color. The etch stop layers enable the accurateremoval of the undesired active regions without harm to the topmostremaining active region in each stack. Then the upper cladding layer andcapping structure are grown over the topmost portions of the structure.Next, a lateral confinement region is created, beneficially usingimpurity-induced layer disordering. Finally, the metallic contacts areadded to each stack and to the substrate.

The advantages of the invention will become more apparent from thefollowing drawings and descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an unscaled schematic depiction of a cross-sectional view of astructure that is in accordance with the invention;

FIG. 2 shows the energy bands for part of the structure in FIG. 1 so asto illustrate the electron and hole populations of the quantum wells;

FIG. 3 shows the conduction band and optical intensities of thewaveguide modes at the two colors output by the second stack of thestructure illustrated in FIG. 1;

FIG. 4 shows an intermediate structure in the fabrication of thestructure illustrated in FIG. 1, that intermediate structure is the onethat exists prior to any etching steps;

FIG. 5 shows the upper part of the structure of FIG. 4 after an etchingstep and deposition of an etch mask;

FIG. 6 shows the structure of FIG. 5 after an etching step and removalof the remaining etch mask;

FIG. 7 shows the structure of FIG. 6 after an etching step;

FIG. 8 shows the structure of FIG. 7 after an etching step; and

FIG. 9 shows the structure of FIG. 8 after growth of layers over theexposed surfaces.

AN ILLUSTRATED EMBODIMENT

The present invention provides for stacked active region monolithicarrays, and techniques for producing such arrays, that are capable ofoutputting closely spaced, multiple color, axially aligned laser beams.A benefit of the present invention is that it enables stacked activeregion monolithic arrays in material systems, such as the Al_(x)Ga_(1-x) As material system, in which high quality growths are difficultto achieve, for example, growths on aluminum containing layers. Anothermaterial system in which the present invention may find wide use isIn_(x) Ga_(y) Al_(1-x) As, wherein 0≦x≦1 and 0≦y≦1.

The following detailed description describes an embodiment of a stackedactive region structure according to the present invention. First, adescription of the structure itself is provided. Then, the operation ofthe structure (as currently understood) is described. Finally, a processof fabricating the structure is provided. It is to be understood thatmany other embodiments than the one illustrated are possible.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

FIG. 1 illustrates a simplified, stacked active region monolithic array8 in accord with the principles of the invention. While the array 8outputs only two laser beams, the principles of the invention can beapplied to arrays which output additional laser beams.

The array 8 is a layered arrangement of Al_(x) Ga_(1-x) As, GaAs, andmetallic contacts. The use of Al_(x) Ga_(1-x) As on GaAs substrates iswell known for the production of lasers having wavelengths in the rangeof around 700 to 900 nm.

The array 8 has a substrate 9 comprised of a GaAs base onto which aregrown (using MOCVD) a 2000Å thick buffer layer 10 of Al₀.15 Ga₀.85 Asfollowed by a 2000Å thick Al₀.40 Ga₀.60 As buffer layer 11, both ofwhich are heavily doped n-type (doping greater than 10¹⁸ cm⁻³). Over thebuffer layer 11 is a 0.8 μm thick Al₀.85 Ga₀.15 As lower cladding layer12 which is doped to about n=10¹⁸ cm⁻³. Because the array 8 outputs twolaser beams, two stacks of active regions and support layers are formedover the lower cladding layer 12. While the subsequently described firststack contains only one active region, and is therefore technically nota stacked active region, it will be referred to as a stack since, inother applications, it may contain more than one active region. As usedherein, an active region is one or more layers capable of providingoptical gain at a color of interest from the array.

THE FIRST STACK

Still referring to FIG. 1, the first stack begins with an approximately900Å thick waveguide layer 14 of Al₀.40 Ga₀.60 As over the lowercladding layer 12. Over the waveguide layer 14 is an 80Å thick quantumwell layer 16 of Al₀.10 Ga₀.90 As (for emission at a first wavelength of780 nm). Over the quantum well layer 16 is a 900Å thick waveguide layer18 of Al₀.40 Ga₀.60 As. The various layers 14, 16, 18 form a firstactive region.

Over the waveguide layer 18 is a 50Å thick etch stop layer 20 that iscomprised of Al₀.15 Ga₀.85 As. Over the etch stop layer 20 is an 0.6 μmthick Al₀.85 Ga₀.15 As upper cladding layer 22 that is doped to aboutp=10¹⁸ cm⁻³. Over the upper cladding layer 22 is a 1000Å thick cappinglayer 24 of GaAs which is doped to about p=2×10¹⁹ cm⁻³. Finally, overthe capping layer 24 is a metal contact 26 that provides electricalaccess to the first stack.

THE SECOND STACK

The second stack begins with an approximately 900Å thick waveguide layer28 of Al₀.40 Ga₀.60 As over the lower cladding layer 12. Over thewaveguide layer 28 is an 80Å thick quantum well layer 30 of Al₀.10Ga₀.90 As. Over the quantum well layer 30 is a 900Å thick waveguidelayer 32 of Al₀.40 Ga₀.60 As. The various layers 28, 30, and 32 form asecond active region. Note that the first and second active regions areessentially the same in that the layers 28, 30, and 32 are the same as,respectively, layers 14, 16, 18. However, as subsequently explained, thesecond active region does not emit laser light.

Over the waveguide layer 32 is a 50Å thick etch stop layer 34 comprisedof Al₀.15 Ga₀.85 As. Note that the etch stop layers 20 and 34 are thesame. Importantly, over the etch stop layer 34 is a 40 A thick etch stop36 comprised of Al₀.85 Ga₀.15 As.

A third active region is then formed over the etch stop 36. That thirdactive region is formed from a 250Å thick waveguide/separation layer 38of Al₀.30 Ga₀.70 As. Over the waveguide/separation layer 38 is a 100Åthick GaAs quantum well layer 40 for emission at 850 nm. Completing thethird active layer is a 900Å thick waveguide layer 42 of Al₀.30 Ga₀.70As over the GaAs layer 40. the etch stop layer 44 is a 6000Å thickAl₀.85 Ga₀.15 As upper cladding layer 46 that is doped to about p=10¹⁸cm⁻³. Over the upper cladding layer 46 is a 1000Å thick capping layer 48of GaAs which is doped to about p=2×10¹⁹ cm⁻³. Finally, over the cappinglayer 48 is a metal contact 50. Note that the upper cladding layer 46,capping layer 48, and metal contact 50 are similar to, respectively, theupper cladding layer 22, capping layer 24, and metal contact 26.

OTHER ELEMENTS OF THE ILLUSTRATED EMBODIMENT

Over the sections of the lower cladding 12 that are not part of eitherthe first or second stacks are lateral confining regions 54. In thearray 8, the confining regions 54 are formed by impurity-induceddisordering. Finally, a common electric contact 56 provides a commonelectrical path for the first and second stacks.

OPERATION OF THE ILLUSTRATED EMBODIMENT

To emit light from the array 8, independent current sources are appliedvia the metallic contacts 26 and 50 through the first and second stacks,respectively, to the common metallic contact 56.

The current through the first stack causes electron-hole recombinationsin the first active region (layers 14, 16, and 18), mainly in thequantum well layer 16. Laser operation takes place on the transitionbetween the confined electron and hole states in the quantum well.Lateral current confinement and lateral optical confinement are providedby the confining regions 54 adjacent the first stack. Additional opticalconfinement is provided by the lower cladding layer 12 and by the uppercladding layer 22. The capping layer 24 provides electrical interfacingbetween the metallic contact 26 and the upper cladding layer 24. Whilenot shown, an optical cavity (formed by cleaved facets) provides therequired optical feedback.

Current through the second stack passes in series through the second andthird active regions. When multiple active regions are stacked over acommon lower cladding layer, with the topmost region having the longestwavelength, the topmost region will lase. This strategy relies on theconcept that carriers preferentially populate the narrowest bandgaplayer in a stacked structure.

A bandgap diagram for the second stack of the structure shown in FIG. 1is presented in FIG. 2. With current flow, electrons and holes populatethe various regions as shown. Because the compositions and dimensions ofthe second stack vary along the stack, electrons and holes, which seektheir lowest energy state, populate the quantum well 40 preferentiallyover the quantum well 30. Because of the much higher electron and holedensities in the lower bandgap quantum well 40 as compared to thequantum well 30, given sufficiently low threshold gain, laser operationtakes place on the transition between confined electron and hole statesin quantum well 40. Only a much smaller amount of spontaneous emissionis emitted from the quantum well 30. Lateral current confinement andlateral optical confinement are provided by the confining regions 54adjacent the second stack.

FIG. 3 illustrates vertical optical confinement in the array 8. For thesecond stack, which has a longer wavelength of operation, an opticalwaveguide mode 62 is achieved as a result of optical waveguiding withinthe waveguide core formed by the layers 28 through 44, inclusive, whichare bounded by the cladding layers 12 and 46. For the first stack, whichhas a shorter wavelength of operation, an optical mode 64 is achieved bythe layers 14 through 20, inclusive, which are bounded by the claddinglayers 12 and 22. It is important that the long wavelength optical mode62 has a relative maximum very close to the long wavelength quantum well(layer 40), and also that the short wavelength optical mode 64 has arelative maximum near the short wavelength quantum well (layer 16).These positions relative to the quantum wells result in maximum overlapfactor of the optical mode with the quantum well for optical modeamplification, resulting in optimal laser threshold characteristics.

It should be noted that the etch stop layers 34 and 44 in FIG. 2 alsoform quantum wells. These quantum wells are not necessary for theoperation of the array 8. They result from the desirability of havinglow-aluminum fraction layers exposed prior to the final growth step (seebelow). The thicknesses and compositions of these layers are chosen suchthat the confined energy states for the electrons and holes in all etchstop layers in a stack lie above those in the quantum well from whichlaser emission is desired. This helps ensure that the electrons andholes preferentially populate the desired quantum well.

FABRICATION

Problems with prior art stacked active region monolithic arrays arefrequently a result of the difficulties of precisely etching to thecorrect location between active regions, and of achieving high qualitygrowths on the topmost surfaces after etching, particularly in materialsystems such as AlGaAs that tend to oxidize. Our solution to theseproblems is to include "etch stop layers" between the stacked activeregions that enable precise etching and subsequent high quality growthsof the desired compositions. It is to be understood that while thefollowing fabrication process describes the fabrication of only threeactive regions (only two of which are stacked) the fabrication processis easily expanded to include more stacked regions.

The initial step in fabricating the structure shown in FIG. 1 is to growthe structure shown in FIG. 4 using a suitable epitaxial crystal growthtechnique such as MOCVD. Referring to FIG. 4, the following layers aregrown, in the order given on the substrate 9 of FIG. 1:

    ______________________________________                                        Layer  Thickness  Composition  Reference Layers                               ______________________________________                                         96    2000 Å Al.sub.0.15 Ga.sub.0.85 As                                                                 10                                              98    2000 Å Al.sub.0.4 Ga.sub.0.6 As                                                                   11                                             100    8000 Å Al.sub.0.85 Ga.sub.0.15 As                                                                 12                                             101     900 Å Al.sub.0.40 Ga.sub.0.60 As                                                                 14, 28                                         102     80 Å  Al.sub.0.10 Ga.sub.0.90 As                                                                 16, 30                                         103     900 Å Al.sub.0.40 Ga.sub.0.60 As                                                                 18, 32                                         104     50 Å  Al.sub.0.15 Ga.sub.0.85 As                                                                 20, 34                                         105     40 Å  Al.sub.0.85 Ga.sub.0.15 As                                                                 36                                             106     250 Å Al.sub.0.30 Ga.sub.0.70 As                                                                 38                                             107     100 Å GaAs         40                                             108     900 Å Al.sub.0.30 Ga.sub.0.70 As                                                                 42                                             109     50 Å  Al.sub.0.15 Ga.sub.0.85 As                                                                 44                                             110    6000 Å Al.sub.0.85 Ga.sub.0.15 As                                                                 None                                           111    1000 Å*                                                                              GaAs         None                                           ______________________________________                                         *not critical                                                            

The reference layers given in the table indicate that those layers inFIG.1 correspond to the layer given in the table. Thus, the layer in thetable must be doped as described with regards to FIG. 1.

Since they will be completely etched away, it is not necessary to growthe layers 110 and 111. However, the layers 110 and 111 facilitatetesting the topmost active region as a single-wavelength emitter priorto the fabrication of a multicolor device. Also, growing the layer 110on top of the layer 109 helps maintain the cleanliness of the topsurface of the layer 109.

After fabrication of the structure shown in FIG. 4, the layer 111 isremoved using a suitable etchant. A suitable etchant is one thatselectively etches layer 111 much more quickly than the layer 110;etchants such as citric acid:hydrogen peroxide or ammoniumhydroxide:hydrogen peroxide having ratios of constituents such that theyetch layer 111 (GaAs) much faster than layer 110 (Al₀.85 Ga₀.15 As) areacceptable. Alternatively, it would be possible to use an etchant thatetches both layers 111 and 110, but with an etch time such that all oflayer 111, but only part of layer 110, is removed. An example of such anetchant is the commonly used etchant system of sulfuric acid:hydrogenperoxide:water.

Referring now to FIGS. 4 and 5, after removal of the layer 111, the topsurface of the resulting structure is patterned, using a suitable etchmask 120, such as a photoresist, with openings where it is desired tolocate the first stack (in FIG. 1, layers 14 through 26 inclusive) andpart of the confinement region 54 on either side of the first stack. Aselective etchant is then used to etch through the exposed layer 110 tothe layer 109, the first etch stop layer. This selective etch could be ahydrofluoric acid based etchant such as buffered oxide etch. The etchmasking layer (photoresist) is then removed, leaving an exposed portionof the layer 109 as shown in FIG. 6.

The remaining and exposed portions of the layer 110 serve as an etchmask for the next etch step. That next etch removes the layers 109, 108,107, and 106, from the area not under the layer 110, see FIG. 7. Thepreferred etchant is a citric acid/water:hydrogen peroxide mixture. Toprepare that mixture, a citric acid solution is formed using equalparts, by weight, of citric acid monohydrate and water. Then, the citricacid/water solution is combined with hydrogen peroxide in a ratio ofcitric acid/water solution to peroxide of 16:1 by volume.

Next, the exposed areas of layer 105 (see FIG. 7) and the remainingportions of the layer 110 are removed using a selective etchant, such asa buffered oxide etchant that etches the layers 110 and 105, but not thelayer 104, see FIG. 8. Layers 122 and 124, similar to the layers 110 and111, respectively, are then grown over the terraced surface formed bythe layers 109 and 104 (in FIG. 8) to complete the epitaxial layerstructure of the monolithic array shown in FIG. 1, see FIG. 9.

The etch stop layers 104 and 109 serve a dual purpose. First, they arebarriers to the etchant that removes the layers above. Second, andimportantly, they serve as relatively aluminum-free surfaces for thefinal growth. If these low-aluminum-containing layers were not included,the growth over exposed layers of higher-aluminum fraction would beproblematic. In the case of the neighboring pair of etch stop layers 104and 105, the lower layer, 104, facilitates the growth, while the layer105 enables selective removal of the active region layers above it.

One particularly appealing feature of the method of fabricating astacked active region monolithic laser array using the method givenabove is that, until the last etching step, the patterning and etchingare carried out such that all regrowth surfaces in the growth plane areprotected by an overlying epitaxial layer. Thus, the risk ofcontamination during the subsequent processing steps is reduced.

After growth of the layers 122 and 124, the confining regions 54 of FIG.1 are formed using impurity-induced layer disordering. First, a layer ofsilicon nitride is deposited over the layer 124. Then, using an etchmask, the silicon nitride layer is removed from the areas into whichsilicon is to be diffused, thereby exposing portions of the layer 124.Next, a layer of silicon is deposited over the exposed portions of thelayer 124 and the remaining silicon nitride layer. Then, another siliconnitride layer is formed over the just deposited silicon. Layerdisordering proceeds by heating the structure in a furnace. The siliconin contact with the layer 124 is driven into the layers below, causingthe desired disordering. Fabrication is then completed by providingseparate contacts for each stack and cleaving to form the opticalcavity.

From the foregoing, numerous modifications and variations of theprinciples of the present invention will be obvious to those skilled inits art. Therefore the scope of the present invention is to be definedby the appended claims.

What is claimed is:
 1. A semiconductor structure, comprising:asubstrate, a lower cladding layer over said substrate; a lower activeregion over said lower cladding layer; a lower etch stop layer over saidlower active region; an upper active region adjacent said lower etchstop layer; an upper etch stop layer over said upper active region; anupper cladding layer over said upper active region; and wherein saidupper and lower cladding layers serve as cladding layers for both saidupper and lower active regions.
 2. The structure of claim 1, furthercomprising:a capping layer over said upper cladding layer; an uppermetallic contact over said capping layer; and a lower metallic contactover the bottom of said substrate.
 3. The structure of claim 2, whereinsaid lower and upper active regions include layers of Al_(x) Ga_(1-x)As, wherein 0≦X≦1.
 4. The structure of claim 2, wherein said lower andupper active regions include layers of In_(x) Ga_(y) Al_(z) As, wherein0≦X≦1, wherein 0≦Y≦1, wherein 0≦Z≦1, and wherein X+Y+Z=1.
 5. Asemiconductor laser structure having an optical resonator, said laserstructure comprising:a substrate, a lower cladding layer over saidsubstrate for providing transverse optical confinement; a lower activeregion over said lower cladding layer, said lower active region having afirst bandgap; a lower etch stop layer over said lower active region; anupper active region adjacent said lower etch stop layer, said upperactive region having a second bandgap which is less than said firstbandgap; an upper etch stop layer over said upper active region; anupper cladding layer over said upper active region for providingtransverse optical confinement; a capping layer over said upper claddinglayer for providing improved electrical contact; an upper metalliccontact over said capping layer for providing a first input terminal tosaid upper and lower active regions; and a lower metallic contact overthe bottom of said substrate for providing a second input terminal tosaid upper and lower active regions; wherein current flow between saidupper and lower metallic contacts passes through said upper and loweractive regions such that said upper active region lases.
 6. Thesemiconductor laser structure according to claim 5, further including aconfinement layer around said lower and upper active regions forproviding lateral carrier and optical confinement.
 7. The structure ofclaim 6, wherein said lower and upper active regions include layers ofAl_(x) Ga_(1-x) As wherein 0≦X≦1.
 8. The structure of claim 6, whereinsaid lower and upper active regions include layers of In_(x) Ga_(y)Al_(z) As, wherein 0≦X≦1, wherein 0≦Y≦1, wherein 0≦Z≦1, and whereinX+Y+Z=1.
 9. A semiconductor laser array having two optical resonators,said laser array comprising:a substrate, a lower cladding layer oversaid substrate for providing transverse optical confinement; a firststack over said lower cladding layer, said first stack comprised of; afirst active region over said lower cladding layer and having a firstbandgap; a first etch stop layer over said first active region; a firstupper cladding layer over said first etch stop layer for providingtransverse optical confinement; a first capping layer over said firstupper cladding layer for providing improved electrical contact to saidfirst upper cladding layer; a first upper metallic contact over saidfirst capping layer for providing an input terminal to said first stack;a second stack over said lower cladding layer, said second stackcomprised of; a second active region over said lower cladding layer andhaving a bandgap equal to said first bandgap; a second etch stop layerover said second active region; a third active region over said secondetch stop layer and having a bandgap which is less than said firstbandgap; a third etch stop layer over said third active region; a secondupper cladding layer over said third etch stop layer for providingtransverse optical confinement; a second capping layer over said secondupper cladding layer for providing improved electrical contact to saidsecond upper cladding layer; a second upper metallic contact over saidsecond capping layer for providing an input terminal to said secondstack; and a lower metallic contact over the bottom of said substratefor providing a common terminal for said first and second stacks;wherein current flow between said first metallic contact and said lowermetallic contact cause the first active region to emit laser light at afirst color, and wherein current flow between said second metalliccontact and said lower metallic contact cause the third active region toemit laser light at a second color.
 10. The semiconductor laserstructure according to claim 9, further including a confinement layeraround said first and second stacks for providing lateral carrier andoptical confinement.
 11. The structure of claim 10, wherein said first,second and third active regions include layers of Al_(x) Ga _(1-x) As.12. The structure of claim 10, wherein said first, second, and thirdactive regions include layers of In_(x) Ga_(y) Al_(1-x-y) As.